The present invention relates to a buzzer system for wrist watches, and more particularly to a buzzer system which is capable of producing an excellent alarm sound by the use of a piezoelectric buzzer element.
The alarm function of a wrist watch is a widely provided and useful function. The requirements for an element for producing an alarm sound are small size, high efficiency and low cost. To satisfy these requirements, the arrangement for producing sound by putting a piezoelectric element directly on part of the casing of the wrist watch, such as the glass or cap, is widely used. However, if the casing of the watch is used as an element for producing sound, since various casings which differ in size or design are prepared for a timepiece module, the resonance point of the glass or the cap depends upon the respective casing. As a result, the suitable resonance point of the glass or the cap is different for every type of casing and it will sometimes occur that the sound pressure level is lowered due to the particular casing used. Moreover, it is disadvantageous that the quality of the sound produced by the use of the casing is uncomfortable.
One of the reasons why the sound is uncomfortable is that the resonance point of the casing or the cap is high and produces a sound having a high frequency component. Another reason is that since the Q-value of the vibrating body is high at its resonance point and the frequency of the free oscillation is different from that of the driving signal, and moreover the frequency ratio therebetween is not simple, the sound is discordant. In addition, for an alarm circuit in which the alarm sound is intermittently cut off in accordance with a square wave signal, at the time of the rising and falling of the signal the rapid change in frequency produces an impulsive or impact sound.
FIG. 1 is a circuit diagram of a conventional piezoelectric buzzer. In this figure, a piezoelectric element 1 and a step-up coil 2 are connected in parallel at the collector of a switching transistor 3. When an alarm signal is applied to the base 4 of the transistor 3 to turn it on or off, the current flows through the step-up coil in accordance with the conductive state of the transistor 3 and the stepped-up voltage is applied to the piezoelectric element 1. For example, when the square-wave signal shown in FIG. 2(a) is applied to the base 4, a voltage having the waveform shown in FIG. 2(b) is applied across the piezoelectric element 1.
The voltage waveform has a resonance characteristic in accordance with the time constant determined by the inductance L of the coil and the capacitance C of the piezoelectric element, and is applied to the piezoelectric element. That is, the fundamental frequency component of the alectric signal for driving the piezoelectric element is 2048 Hz when the frequency of the input signal applied to the base 4 is 2048 Hz, and higher harmonic components which are integer multiples of the fundamental frequency are included. The magnitude of the higher harmonic components depends upon the values of L and C.
Generally, it is difficult to reduce the dispersion of the inductance value of the coil in view of the method for manufacturing it. Moreover, to prevent the piezoelectric element from changing due to a temperature change, or to allow it to be incorporated into the various types of casings, the piezoelectric element configuration cannot be predetermined, so that it is impossible to maintain the capacitance value thereof constant. Therefore, it is necessary to increase the manufacturing cost so as to keep the L-C resonance point constant for every device.
In the circuit shown in FIG. 1, the sound pressure level is widely changed because changes in the characteristic of the sound-producing body are superposed on the changes in the frequency components of the driving signal.
An alarm watch having a piezoelectric vibrator connected to the watch case to produce an alarm sound is disclosed in U.S. Pat. No. 3,733,804. This patent also discloses associated circuitry comprised of an oscillator circuit and a voltage multiplier for driving the piezoelectric vibrator. The alarm sound is produced at a predetermined time by controlling energization of the oscillator circuit by simply turning on the power used to energize the oscillator circuit. Similarly, the alarm sound is terminated by turning off the power applied to the oscillator circuit. No provision is made for changing the frequency of the oscillator circuit or for changing the amplitude of the oscillator output signal in a controlled manner during the course of producing the alarm sound.
U.S. Pat. No. 3,759,029 discloses an electronic timepiece with a time signalling device which does include circuitry for changing the frequency of an alarm sound. This circuitry is comprised of an oscillator circuit and a frequency divider stage comprised of a plurality of flip-flops connected in series. Consequently, the resultant ratio between possible frequencies is limited to values which are an integer multiple of two.